In the example of flexible transmission machines, such a control system is used for controlling the operating speed of tools disposed at the end of the flexible transmission. Since the user's hands are often occupied elsewhere, such control systems organized to have a pedal for control by the foot. However, such foot control does not provide great accuracy and the tooling often includes additional hand-adjusted control means for setting the operating speed of tools more accurately. The need to provide two distinct control means enabling the operating speed of tools to be varied is disadvantageous both with respect to the ease with which the tools can be used and with respect to the cost thereof.
To overcome this drawback, American patent U.S. Pat. No. A-4 299 182 proposes a control system including a pedal acting on a rheostat, and a series of buttons enabling circuits comprising discrete resistances to be selected to replace the rheostat. However, such buttons are not capable of enabling continuous adjustment of the controlled parameter to be obtained by hand. In addition the circuit included in that control system is relatively complex since it is necessary to provide a large number of discrete resistances and of switching means in addition to the rheostat conventionally included in pedal control means.
U.S. Pat. A-4 284 018 discloses a sewing machine control system in which a conventional pedal means can be slid inside a box that includes hand control means enabling the pedal to be adjustably pressed down. Although that control system does indeed enable continuous and accurate manual control to be obtained, it is not optimum with respect to cost and ease of use since it requires a box in addition to and distinct from the pedal control means.